The present invention relates to a source follower circuit using at least one field effect transistor (hereinafter referred to as an FET), or more in particular to a source follower circuit using at least one power MOSFET (Metal Oxide Semiconductor FET).
As compared with a bipolar transistor, a MOSFET is high in the gain and is therefore effective in simplifying the circuit configuration. The many advantages of MOSFET also include superior frequency characteristics and fast switching operation, and hence less distortion. The source follower circuit, on the other hand, has especially superior frequency characteristics.
Conventional source follower circuits using MOSFETs, however, are used to amplify small signals to generate only a comparatively small AC signal and are used for amplification of, say, the front end of an FM tuner.
The input signal source for these circuits supplies an AC signal of at most several volts, in which the capacitance between drain and source of a used MOSFET is not more than several pF, so that the MOSFET is not subjected to an undesirable oscillation. In the power MOSFET, however, the input signal source thereof supplies an AC signal of several tens of volts so that the capacitance between the drain and source of a used MOSFET is as large as several hundreds pF, which often causes the MOSFET to oscillate.
The power MOSFET is provided with a radiation fin for radiating heat, with the result that the wiring connecting the gate terminal for signal input and the printed circuit board is considerably long. Further, the wiring between the drain and the speaker making up a load is undesirably long. These wiring conditions often cause the oscillation of the power MOSFET.
An object of the present invention is to eliminate or suppress the above-mentioned disadvantages of the prior art and to provide a stabilized source follower circuit using at least one FET free from oscillation. More particularly, it is an object of the present invention to provide an FET source follower circuit which may be used stably in the power stage without any oscillation.
To achieve this object, according to the present invention, a resistor is inserted between the gate terminal of an FET with a signal source, and the resistance of the resistor being determined to be at such value as to prevent oscillation of the FET.
According to one aspect of the present invention, there is provided a source follower circuit using at least one FET having a gate terminal, a drain terminal and a source terminal. The source follower circuit comprises a load resistor with one end thereof connected to the source terminal, a power supply connected for applying a predetermined voltage between the other end of the load resistor and the drain terminal, a signal source with one end thereof connected to the other end of the load resistor, and a resistor connected in series between the signal source and the gate terminal.